First, NEVER tie hots together, for many reasons. Not the least of which is that they could easily be on different phases and result in an effective short. But you can split your load between two hots.. some equipment on one and some on the other.
So then.. what about the ground and the neutral...
If you were to use only one neutral, you'd have one problem or another. If the circuits are on the same leg/phase, you would be running twice as much current through the neutral as it is rated for, resulting in a dangerous situation. If the hots are on different legs, you would not be running as much neutral current, but you'd run the risk of, if your neutral plug fell out, you'd have (in the USA) either 220V or 208V across your gear in a series connection with the neutral floating somewhere in between! Not a good situation at all. A loose neutral on a multi-leg/phase circuit is a scary thing. So no, never use just one neutral in this way.
So, you have two choices.. you can tie the neutrals together, or use them separately paired with their hots. I suggest the latter if you can.
So then, the ground. If you use separate grounds, you run the risk of ground loops. If you bond them, then you bypass that possible ground current around your gear, but you still have ground current, which isn't good. If you lift one ground and use the other for all the gear, that's not bad.. until the cord with the one ground falls out and you're left with powered-up gear with no ground. Again, a dangerous situation.
So, really, there is no absolutely right answer here, thus why it's actually not legit to do these things. But sound guys need the power, and the client is waiting.. so these things get done.
I will tell you how I do it. I use a single-phase distro in my rack, two legs. Normally this is tied straight into a breaker panel. But when I don't have that option, I use two outlets, on two circuits, always from the same panel, with both neutral and ground bonded and hots separate. If I am not sure, I check with a voltmeter between the neutrals on the two outlets, and between the grounds on the two outlets. If there is more than a small (ideally a volt or two) difference between them then I don't bond them and I have to find other outlets. I also use an outlet tester to make sure the ground is good on both outlets and that the neutral and hot are not reversed. If they were reversed then my plugging in my bonded adapter would cause a short circuit. Since my distro expects a single neutral and a single ground, I have no choice but to find ones that I can safely bond.
So, yes, you can. And yes, there's a reason it's not really considered ok. Use caution. If you don't understand, then just don't.
A safer approach is to simply divide your gear between circuits and use audio isolation transformers to break any ground loops that cause a problem (eg the ART DTI is pretty good for a low price, the Whirlwind ISO-2 is better). For example run the amp rack on one circuit and everything else on another.. or subwoofer amps on one circuit, everything else on another.. with an iso box between them. It is much easier and safer.